The present invention relates to electrically conductive composites based on a mixture of two thermoplastic compounds having different polarity with fibers and /or mineral filler and carbon black and to methods for producing same.
Electrically conductive polymer based compositions are used in many industrial applications, such as for dissipating electrostatic charge from plastic parts and plastic boxes for shielding electronic components from electromagnetic interference (EMI). Examples for electrostatic discharge (ESD) applications are electronic packaging, clean room installations, storage trays, water carriers, chip carriers and construction components for explosion-proof environments.
Compounds tailored for dissipating static electricity have a typical surface resistivity of 102 to 1013 ohm/square and compounds specified for EMI shielding applications typically exhibit volume resistivity of 10xe2x88x922 to 102 ohm-cm.
There are known in the art polymer based compounds having appropriate resistivity for both static electricity dissipation and EMI shielding. One class of compounds known in the art is based on polypropylene (PP) or polyethylene (PE) with high carbon loading levels up to 40 to 60% by weight characterized by surface resistivity of about 103-108 ohm/square. For applications requiring EMI shielding, weight loading levels of 30 to 50% PAN carbon fibers, 40% aluminum flakes, 15% nickel-coated carbon fibers or 5 to 10% stainless steel fibers have been used for the same class of polymers.
The method currently used to increase the electrical conductivity of polymers is to fill them with specific conductive additives, such as metallic powders, metallic fibers, carbon black, carbon fibers and recently with intrinsically conductive polymeric powders. The characteristic behavior of these materials is the existence of a strongly non-linear relationship between the electrical conductivity and the filler concentration. At low filler loading, the electrical conductivity of the polymeric compound is generally quite low; its magnitude is similar to that of the polymer matrix (10xe2x88x9216 to 10xe2x88x9211 ohmxe2x88x921 cmxe2x88x921). As loading is increased, the conductivity increases sharply by several orders of magnitude over a narrow concentration range, then slowly increases towards the conductivity of the condensed filler powder on the order of 10xe2x88x924 to 10xe2x88x921 ohmxe2x88x921 cmxe2x88x921. This behavior describes an insulator-conductor transition occurring at a critical volume fraction (percolation threshold). This threshold is due to the formation of a chain-like network of particles extending throughout the entire specimen volume and allowing electrical current to flow.
U.S. Pat. No. 4,169,816 describes an electrically conductive single thermoplastic material composition with a high carbon content, the composition including for each 100 parts of polypropylene-ethylene copolymer 15-30 parts of carbon black, 0.25 to 1 part of silica and 1-10 parts of a fiber reinforcing agent selected from carbon fibers or a mixture of carbon fibers and glass fibers.
U.S. Pat. No. 5,004,561 describes another single thermoplastic based electrically conductive composition with a high carbon content, the composition including for each 100 parts of thermoplastic resin selected from the group of polyolefin, polystyrene and acrylonitrite/styrene/butadiene (ABS) copolymer resin, polybutylene terephthalate (PBT) resin, polyphenylene ether and polyamide (PA) resin, 30-300 parts of electrically conductive glass fibers, 5-40 parts of carbon black and 5-40 parts of graphite.
Russian Patent No. SU 1,643,568 describes a high carbon based electrical conductive thermoplastic composition in which electrical conductivity is achieved from the dispersion of carbon in the matrix. The composition includes 20-35 weight percent polypropylene, 10-20 weight percent polyamide, 20-30 weight percent carbon black, 10-20 weight percent graphite and 15-20 percent glass fibers.
There are generally two methods for producing electrically conductive thermoplastic articles known in the art. In the slow production rate compression molding method less filler (e.g. carbon black) is required to achieve a desired conductivity, however the mechanical properties of the composition are usually deficient. In the fast production rate injection molding method better mechanical properties are achieved and articles having complex geometry can be produced but the amount of conductive filler required is high. One deficiency of compression molding of electrically conductive compounds is that the relatively slow processing is expensive.
A major disadvantage of prior art polymer based compounds for electrostatic dissipation and EMN shielding applications is the high percentage of conductive additives required to form the conductive polymer compounds resulting in high cost and deficient processability and mechanical properties and also high carbon contamination which is adverse in particular for clean room applications.
The present invention provides an improved thermoplastic electrically conductive composition.
According to an aspect of the present invention, the electrically conductive composition includes a first thermoplastic component forming a continuous matrix and a second thermoplastic component having a polarity larger than the polarity of the matrix. The composition also includes fibers being encapsulated in-situ by the second thermoplastic component and forming a network within the matrix and also a carbon black component which is preferentially attracted to the second component due to its higher polarity. The in-situ formation of an encapsulated network, including carbon black, in preferred locations of particles provides an electrically conductive composition.
According to a further aspect of the present invention, the ratio between the conductive carbon filler and the second component is sufficiently high so that a substantial part of the carbon filler is located at the interface between the second component and the matrix to provide the electrical conductivity. Nevertheless, the overall concentration of carbon is at least an order of magnitude smaller than in the prior art electrically conductive compositions, thus making the compositions of the present invention advantageous in many applications including clean room applications.
Another object of the present invention is to utilize a fast processing method for producing the thermoplastic electrically conductive compositions of the present invention. Injection molding is used for producing the electrically conductive thermoplastic compositions of the present invention while using very low carbon black concentrations and improving the mechanical properties of the composition.
The electrically conductive composition of the present invention includes a matrix including substantially a first thermoplastic component, a second thermoplastic component having a higher polarity than that of the first thermoplastic component, the second component encapsulating a plurality of fibers forming a network of encapsulated fibers within the matrix, and a carbon component preferentially attracted to the second component so as to make the network an electrically conductive network within the matrix.
In one embodiment of the present invention, the first thermoplastic component is a polyolefin compound with or without an added elastomer component. The polyolefin is selected from the group of polypropylene which may be a homopolymer or a copolymer and polyethylene. The second component is polyamide or EVOH. In a preferred embodiment, the composition includes less than 20 parts per hundred polyamide or EVOH.
In another embodiment, the first component is acrylonitrite/butadiene/styrene and the second component is polyamide, or EVOH.
In yet another embodiment, the first component is selected from polystyrene, high impact polystyrene and polyphenyleneoxide/ polystyrene and the second component is polyamide or EVOH.
In another embodiment the first thermoplastic component may be polybutylene terephthalate, polycarbonate or polycarbonate acrylonitrile butadiene styrene.
In yet another embodiment the second thermoplastic component may be acrylonitrile butadiene styrene.
The fibers of the compositions of the present invention may be glass fibers. In a preferred embodiment, the composition includes less than 55 parts per hundred glass fibers.
The second component may also encapsulate filler particles, such as mineral filler, organic fibers, such as polyamide fibers, or a mixture of filler and glass fibers.
The carbon component of the compositions of the present invention may be carbon black. Alternatively, or in combination, the carbon component may be carbon fibers. In a preferred embodiment, the composition includes less than 10 parts per hundred carbon black. In another preferred embodiment, the composition includes less than 30 parts per hundred carbon fibers.
In a preferred embodiment, the composition has one or more of a volume resistivity from about 0.1 to about 109 ohm-cm, a flexural modulus of up to about 11,000 MPa, and a tensile strength of up to 60 MPa.